petroleum geology 10 11

8/3/2019 Petroleum Geology 10 11

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Exam Calendar

20/6/2011, ore 9.00, Aula E, Dip. Scienze della Terra


15/7/2011, ore 9.00, Aula E, Dip. Scienze della Terra 29/7/2011, ore 9.00, Aula E, Dip. Scienze della Terra




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PETROLEUM

GEOLOGY

TEXT BOOKS:

Geology of Petroleum, second edition, by A.I. Levorsen

Petrolio, M. Pieri

Petroleum Geology eTextbook


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Summary

Origin and accumulation of Organic Matter (OM)

Preservation of OM

Transformation of OM to kerogen Diagenesis, catagenesis, metagenesis

Migration

Trapping


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How did petroleum most likely form?

Petroleum is primarily made from plankton. Phytoplankton(plants) and zooplankton (animals) are microscopic organisms

that live in water, both freshwater lakes and the oceans. These

creatures die and sink to the bottom of the ocean where they

accumulate in the sediment along with organic materials that are

washed into the lake or ocean by rivers and streams.


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THE PRODUCTION AND ACCUMULATION

OF ORGANIC MATTER

Organic matter is defined as material

comprised solely of organic molecules in

monomeric or polymeric form, that are derived

directly or indirectly from the organic part of

organisms. deposited or preserved in

sediments

The phytoplankton is the largest producer of

organic carbon in water. The contribution of

larger marine organisms is negligible.

The production of organic matter occurs in the

euphotic zone (the first 60-80 m of water depth).This production can reach 600g/mq/yr, for a

total amount of about 60 billion tones a year.

Only the 0.1 to 0.01% of the organic matter

produced becomes fossil fuel.


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CHEMICAL COMPOSITION OF LIVING MATTER

The organic matter that accumulates in sedimentsbelongs to the following groups of chemical

constituents:

Lipids

Protein

Carbohydrates

Lignin (higher plants)


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Environments where bioproductivity and depositional

environment favour the accumulation of organic rich

sediments.


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DIAGENESIS(from 0 to several hundreds of metres, about 60 C)

CATAGENESIS(several kilometres, about 150 C)

METAGENESIS(more than 5-6 km, more than 150-200 C)

TRASFORMATION OF ORGANIC MATTER

Three distinct stages can be recognized based onthe degree of alteration of the organic matter:


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In high porosity rocks: aerobic bacteria utilize the oxigen

occurring in the water to degrade the organic matter

(oxidation) and generate H2O, CO2.

In low porosity rocks:

anaerobic bacteria mustacquire oxygen via a sulfate

reduction process, which is a

relatively slower process

(fermentation) and generateCO2 e CH4 (biogenic gas).

Through the disintegration of

organic matter, reactive

molecules are produced thatcombine to formgeopolymers


(kerogen).


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Trasformed inbiogenic gas through

fermentation



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CATAGENESIS (several kilometres, about 150 C)

The onset of petroleum generation and the thermal degradation of kerogen

marks the beginning of catagenesis. The kerogen is a macromolecule composed

primarily of carbon (C) and hydrogen (H) and to a lesser extent, oxygen (O),sulfur (S) and nitrogen (N).

The classification of different

types of kerogen is based on the

values of the atomic ratios H/Ce O/C. During the

catagenesis, the kerogen is

heated and becomes relativelyricher in carbon releasing

lighter molecules of CO2, H2O

and crude oil or natural

gas, collectively known ashydrocarbons.

Diagram of van Krevelen

Mixed terrestrial and

marine source materialthat can generate

abundant oil and gas

Woody terrestrial source

material that typically

generates gas

Rare, produces high

amounts of oil


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Thermal maturation is the natural

transformation of kerogen into

petroleum in response to increasedthermal stress, which is due to an

increase in burial depth

throughout the geological time.

The oil window is a

temperature dependant interval inthe subsurface where oil is

generated and released from the

source rocks. The oil window is

often found in the 65-125 degree

Celsius interval (aprox. 2-4 kmdepth), while the corresponding

gas window is found in the 100-

200+ degree Celsius interval (3-6

km depth).

THE OIL WINDOW


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CATAGENESIS (several kilometres, about 150 C)

METAGENESIS

Graphite


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POROSITY

Pore volume X 100Total volume of the rock

Total porosityEffective

Non-effettive

The percentage of pore volume or void space, or that volume within rock that can contain

fluids. Porosity can be a relic of deposition (primary porosity, such as space betweengrains that were not compacted together completely) or can develop through alteration of

the rock (secondary porosity, such as when feldspar grains or fossils are preferentially

dissolved from sandstones).

Effective porosity is the interconnected pore volume in a rock that contributes to fluid

flow in a reservoir. It excludes isolated pores.

Total porosity is the total void space in the rock whether or not it contributes to fluid flow.Thus, effective porosity is typically less than total porosity.


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POROSITY

PrimaryIntergranular

Intragranularsedimentation

TYPE ORIGIN

Secondary

*Diagenetic

**Vuggy

***Moldic

Fractures

replacement

solution

tectonic

*Diagenetic. The geochemical process where magnesium [Mg] ions replace calcium [Ca] ions in calcite, forming the

mineral dolomite. The volume of dolomite is less than that of calcite, so the replacement of calcite by dolomite in a

rock increases the pore space in the rock by 13% and forms an important reservoir rock. Dolomitization can occur

during deep burial diagenesis.

**Vuggy porosity is pore space that is significantly larger than grains or crystals

***Moldic porosity. A type of secondary porosity created through the dissolution of a preexisting constituent of a

rock, such as a shell, rock fragment or grain. The pore space preserves the shape, or mold, of the dissolved material


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PRIMARY POROSITY IN CLASTIC ROCKS

Effect of sorting onto the interparticle porosity: Poorly sorted sedimentsare less

porous than well sorted sediments. Sorting is an important characteristic of

siliciclastic reservoir rocks because as the degree of sorting decreases, the interstitialarea in between the large grains becomes increasingly infilled with finer material.


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Make a comment to this slide.

How does porosity change?


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A diagrammatic representation of the basic fabric-selective porosity types used in

the Choquette and Pray (1970) carbonate porosity classification. What is meant by

fabric selectivity is that the porosity is controlled by the grains, crystals, or other

physical structures in the rock and the pores themselves do not cross those primary

boundaries.

Choquette & Pray (1970) basic, fabric-selective porosity types


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A diagrammatic representation of the basic non-fabric-selective or variably fabric-

selective porosity types used in carbonate porosity classification. These are all porosity

patterns that actually or potentially can cross-cut primary grains and depositional

fabrics. They also include porosity types that potentially can be much larger than any

single primary framework element.

Choquette & Pray (1970) basic nonfabric-selective or

variable porosity types


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Primary porosity (interparticle). The primary porosity in this sandstone

partially filled with quartz cement during diagenesis. The remaining primaryporosity, filled with blue epoxy during sample preparation, has not been altered

by diagenesis. The field of view is approximately 0.65 mm wide


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Shelter porosity. A type of primary interparticle porosity created by the

sheltering and umbrella effect of relatively large sedimentary particles

which prevent the infilling of pore space by finer clastic particles.

I t d i t ti l it


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Inter- and intra-particle porosity

Intra-particle porosity

Inter-particle porosity


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Moldic porosity. Moldic porosity is a type of secondary porosity

that preserves the shape of the precursor grain, such as these

fragments of fossils. Pores were filled with blue epoxy during

sample preparation.


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Close-up photograph ofvuggy porosity These

features indicate pervasive karstification.


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The whitish-blue dolomite dolomite crystals are replacing the pink-

stained calcite through the process of dolomitization. The field of view is

approximately 2 mm wide.

Dolomitization. The geochemical process where magnesium [Mg] ions replace

calcium [Ca] ions in calcite, forming the mineral dolomite. The volume of

dolomite is less than that of calcite, so the replacement of calcite by dolomite in

a rockincreases the pore space in the rock by 13% and forms an important

reservoir rock. Dolomitization can occur during deep burial diagenesis.


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Fractureporosity

A type of secondary porosity produced by the tectonic fracturing

of rock. Fractures themselves typically do not have much

volume, but by joining preexisting pores, they enhance

permeability significantly. In exceedingly rare cases, nonreservoir

rocks such as granite can become reservoir rocks if sufficient

fracturing occurs


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Fractureporosity


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PRIMARY, SECONDARY, and

TERTIARY MIGRATION


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Petroleum System Elements

Source Rock - A rock with abundant hydrocarbon-prone organic matter.

Reservoir Rock - The hydrocarbons are contained in a reservoir rock.

This is a porous rock. The oil collects in the pores within the rock. The

reservoir must also be permeable so that the hydrocarbons will flow to

surface during production.

Seal or Cap-Rock - A rock through which oil and gas cannot moveeffectively. Attributes which favour a rock as a seal include a small pore

size, high ductility, large thickness, and wide lateral extent.

Trap - The structural and stratigraphic configuration that focuses oil andgas into an accumulation.

Migration RouteAvenues in the rock through which oil and gasmove from source rock to trap (faults, fractures, permeable rocks).


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Diagram of a cross section of a petroleum bearing

basin, illustrating the five key components: source

rock, seal, reservoir, trap, and migration route


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MIGRATION

Oil and gas migrate as:

SMALL GLOBULES or DROPS (larger than a micron)

TRASPORTED BY TERMOGENIC GAS (only light oils)

IN WATER SOLUTION (particularly gas)

Migrate because of:

Hydrodinamic pressure

Compaction


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Transformation of kerogen into products of lower density produces an increase

of volume and, therefore, of pressure and generates a network of microfractures

PRIMARY MIGRATION

(within the source rock up to the contact with a different, more permeable rock


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PRIMARY MIGRATION

(within the source rock up to the contact with a different, more permeable rock)

Overpressure zones


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SECONDARY MIGRATION

(through an integrated system composed of permeable rocks, fractures, and faults)

Oil and gas are less dense than water and, following expulsion from the source

rock, they rise towards the Earths surface unless movement is arrested by aseal. Seals tend to be fine-grained or crystalline, low-permeability rocks, such

as mudstone/shale, cemented limestones, cherts, anhydrite, and salt (halite).

Seals can also develop along fault planes, faulted zones, and fractures. The

presence of seals is critical for the development of petroleum pools. In the

absence of seals, hydrocarbons will rise to the Earths surface as oil seeps, and

be destroyed.

oil seep


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Fault

Faults can act as both conduits

(migration pathways) and seals,

depending on the hydraulic conditions,

the rock properties of the faults, and the

properties of the rocks juxtaposed

across the faults. The consideration of

faults as seals follows the same

reasoning as for cap-rock seals above,

i.e. the sealing capacity of a fault relatesto its membrane strength and hydraulic

strength. Fault seals fail when the

pressure of the petroleum can exceed

the entry pressure of the largest pores

along the fault plane.


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CONTROLLED BY:

buoyancy pressure (spinta di galleggiamento)

capillarity pressure (pressione capillare)

hydrodynamic pressure (pressione idrodinamica)

Water density is in the order of 1-1.2; that of oil is di 0.7-1; that of

gas < 0.001

Thebuoyancy pressure increases in proportion to the density

difference between water and oil and with the diametre of

globules so that very small globules may have a compulsion

insufficient to lift.

SECONDARY MIGRATION



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SECONDARY MIGRATION

(throug a system composed of permeable rocks integrated with fractures and faults)

The termination or

continuation of movement is

determined by an interplay

between the driving force

(buoyancy pressure) and the

resistive force (capillarity

pressure).

When the upper and lower radii (r) within the distorted

globule globule are equal to one another, the capillarity

force is overcome and the globule can rise due to buoyancy


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The balance between buoyancy and capillary pressure of a

sealing rock is given by:

2(1/rt1/rp) = ho g (w-o)

(Capillarity pressure) (Buoyancy pressure)

= interfacial tension between oil and water (dine/cm)

rt = globule radius outside the pore (cm)

rp = globule radius inside the pore (cm)

ho = height of the column of oil (cm)

g = acceleration of gravity (cm/s2)

w = water density (g/cm3)

= oil density (g/cm3)

SECONDARY MIGRATION



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A pressure vs. depth plot, illustrating

a typical water gradient (aquifer)

supporting a petroleum column,

whose steeper gradients lead to a

pressure difference (Pb, buoyancy

pressure) at its maximum beneath the

seal

SECONDARY MIGRATION



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Fluid content

The pores of a reservoir rock are filled by water, oil and gas which

may be still or moving.

Variations in pressure,

temperature, density

and volume can put in

motion these fluidsalong gradients that

will move from places

with high potential

energy to places of

lower energy.

A. di fondo

A. marginale


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The buoyancy pressure is

proportional to the height of the

column of oil that accumulates

under the cover. The maximum

height of the column of oil that can

be held in a combination ofreservoir and rock cover is called

critical height(ho).

The seal capacity determines the

height of a petroleum column that

can be trapped beneath it, and the

seal will be breached when the

buoyancy pressure (Pb) exceeds the

seal capillary entry pressure (Pd).

SECONDARY MIGRATION


ho=2 (1/rt1/rp)

g (w-o)


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The distance traveled by the oil from the source rock to the

reservoir varies from case to case and, in the case of large fields,

may be of the order of tens of kilometers.

During migration, the composition of the mixture that makes up

the oil changes (splitting), becoming enriched of the lightest and

mobile fractions (saturated hydrocarbons and, to a lesser extent,

aromatic hydrocarbons)

SECONDARY MIGRATION



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Tertiary migration includes leakage,seepage, dissipation, and alteration of

petroleum as it reaches the Earths surface.

Tertiary MIGRATION



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PETROLEUM TRAPS


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Petroleum System Elements

24803

Source RockSource Rock

Top Seal RockTop Seal Rock

Reservoir RockReservoir Rock

Anticlinal TrapAnticlinal Trap

(Organic Rich(Organic Rich)

(Impermeable)

PotentialMigration Route

(Porous/Permeable)


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The exact definition ofconventional and unconventional

reservoirs is vague. In economic terms, a conventional

reservoir is one in which a reasonable profit can be made at low

oil or gas prices and without requiring large volume stimulation

prices and without requiring large volume stimulationtreatments. Likewise, an unconventional reservoir can be

described as one that requires the higher oil and gas prices and

large volume treatments before a reasonable profit can be made.


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Trap is the term to describe the body, bounded by seals and containing reservoir, in which

petroleum can accumulate as it migrates from the source rock to the Earths surface. There are

many different trap geometries. These can be grouped into two categories:

Static traps:

Structural (anticlines, faults)

Stratigraphic (primary and secondary)

Traps that combine both elements (mixed)

Dynamic traps:

the closure is provided by the idrodynamic flux that contrasts the buoyancy pressure

Structural traps are created by tectonic, diapiric, compactional, and gravitational processes.

Almost the entire worlds discovered petroleum is in traps that are largely structural.

Stratigraphic traps are formed by lithological variations or property variations generated by

alteration of the sediment or fluid through diagenesis. Much of the worlds remainingundiscovered petroleum will be found in stratigraphical traps.

Purely hydrodynamic traps are rare. Such traps rely on the flow of water through the reservoir

horizon to drag the petroleum into a favourable trapping configuration, such as the plunging

nose of a fold.

.

TRAP CLOSURE


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Closure.The vertical distance from

the apex of a structure to the lowest

structural contour that contains the

structure. Measurements of both theareal closure and the distance from

the apex to the lowest closing

contour are typically incorporated

in calculations of the estimated

hydrocarbon content of a trap.

In map view (top), closure is the

area within the deepest structural

contour that forms a trapping

geometry, in this case 1300 ft [390

m]. In cross section A-A', closure is

the vertical distance from the top of

the structure to the lowest closing

contour, in this case about 350 ft

[105 m]. The point beyond which

hydrocarbons could leak from or

migrate beyond the trap is the spillpoint.

TRAP CLOSURE


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Crest

Spill plain

Closure

Splill point

Trap volume

The trap capacity


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The trap capacity

The following properties must be known or estimated in order

for the petroleum volume to be calculated:

1. Net to gross

2. Reservoir effective porosity

3. Permeability

4. Petroleum saturation

5. Reservoir thickness

6. Trap closure

H d b T T


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Hydrocarbon Trap Types

American Petroleum Institute, 1986

Salt DomeFault

Unconformity

Pinchout

Anticline


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The Basic Trap

Gas

Oil

Water

Petroleum Accumulates in Anticlines

St t l t


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Structural traps

Piercement

Fold

Fault

Combination

fold/fault

F ld d i t t t l t


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Fold-dominant structural traps

Fold dominant structural traps


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Fold-dominant structural traps

Fault dominant structural traps


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Fault-dominant structural traps



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Combination fault fold structural traps


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Combination fault-fold structural traps



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Piercement structural traps


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A salt dome begins to form when a small

part of a wet salt layer rises. That causes

other salt in the layer to flow horizontally

and then up into a rising plume. If the salt

is abundant and saturated with water,

friction offers little resistance, and salt willcontinue to feed into the rising plume. The

upturned (or bowl-shaped) layers next to

the salt dome can become traps in which

oil collects, so understanding salt domes

has great economic value.



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Stratigraphic traps: due to lateral changes of

permeability produced by lateral changes in grain-

size

primary

secondary

Secondary stratigraphic traps form as a


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Oil-water contact

Oil-gas contact

y g p p

consequence of cementation or dissolution, but,

usually, they are due to the occurrence of

unconformities


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Primary and secondary stratigraphic traps


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Dynamic traps


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Dynamic traps


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dZ/dl=w/(w-o) x dh/dl


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An oil play - A particular

combination of reservoir, seal,

source and trap associated withproven hydrocarbon

accumulations.

An oil pool - A subsurface oil

accumulation.

An oil field - Consists of one or

more oil pools.

Principles of seismic


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surveying: (A) onshore,

with diagrams showing the

various methods of

producing the signal, and

(B) offshore

Principles of seismic surveys. P = P-wave, S = S-wave


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3D Seismic Image of a continental margin


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g g


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Raster Box display


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Three Planes display


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Three Planes display


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Random Profile display


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Primary recovery


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Reservoir fluids (gas, oil, water) are under high pressure (geostatic

and hydrostatic pressure) and elevated temperature, any drop inpressure, such as opening the borehole to near atmosferic pressure,

will result in an increase in volume, producing flow. Removing a

volume of fluid will also lead to drop in pressure. However, the

amount of pressure drop depends upon the type of fluid. Gas is

highly compressible, so removing a small amount of gas will not

appreciably affect reservoir pressures. In contrast, oil is not very

compressible, so removing oil will create a measurable drop in

reservoir pressure, unless the volume removed is replenished by

another fluid (e.g., water).

y y

The natural expansion of reservoir fluids is the

primary energy source for initial production

The recovery efficiency depends on the nature of mechanisms

providing pressure to the pool.

Primary recovery


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Simple expansion: Oil and water are not very

compressible, so that drop in reservoir pressure, does

not generate a significant increase in oil volume.

y y

5-6% liquids, more than 90% gas

Primary recovery


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y y

There are two basic types of primary drive

mechanisms: gas drive and water drive.

Gas drive. There are two types of gas drive

mechanisms: gas cap and solution gas (depletion)drive. Both mechanisms function through the

expansion of gas and the volumetric displacement of

oil. The difference between them is the presence or

absence of an initial gas cap.

Primary recovery


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y y

Thegas cap drive is a reservoir containing free-gas in the highest point of the

trap, as a gas cap. Reservoir pressure is maintained by expansion of the gaswithin the gas cap.

Thegas solution (depletion) mechanism lacks an initial free gas cap. A pressure

drop, due to initialwithdrawal of oil from the reservoir causes gas to come out of

solution . The dissociation and expansion of gas drives the oil. The moviment of

the gas within the reservoir will be generally upwards towards the crest of the

trap to form a gas cap.

20-40% of liquids

Very high for gas, 5-30% liquids

Gas cap expansion

Primary recovery


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y y

The water drive: the water drive mechanism occurs because the

natural hydrodynamic flow into the structure maintains pressurebeneath the pooled oil, driving the oil upwards. A natural water

drive mechanism occurs when the underlying aquifer is large and

capable of undergoing recharge.

35-70% liquids

Secondary recovery


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Over the lifetime of the well the pressure will fall, and at some point

there will be insufficient underground pressure to force the oil to the

surface. After natural reservoir drive diminishes, secondary recoverymethods are applied. They rely on the supply of external energy into

the reservoir in the form of injecting fluids to increase reservoir

pressure, hence replacing or increasing the natural reservoir drive with

an artificial drive. Secondary recovery techniques increase the

reservoir's pressure by water injection, natural gas reinjection and gas

lift, which injects air, carbon dioxide or some other gas into the bottom

of a production well, reducing the overall density of fluid in the

wellbore. Typical recovery factors depend on the properties of oil and

the characteristics of the reservoir rock. On average, the recovery factorafter primary and secondary oil recovery operations is between 30 and

50%

y y

Secondary oil recovery


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Gas injection

y y

Gas injection is presently the most-commonly used approach to enhanced recovery. Unwanted gas is

injected into the gas cap or oil-bearing stratum under high pressure. That pressure pushes the oil into

the pipe and up to the surface. In addition to the beneficial effect of the pressure, this method

sometimes aids recovery by reducing the viscosity of the crude oil as the gas mixes with it.

Gases commonly used include CO2, natural gas or nitrogen.

Secondary oil recovery


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Water flood

y y

Tertiary (enhanced) oil recovery


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The third stage of hydrocarbon production during which sophisticated techniques that alter the

original properties of the oil are used. Enhanced oil recovery can begin after a secondary

recovery process or at any time during the productive life of an oil reservoir. Its purpose is not

only to restore formation pressure, but also to improve oil displacement or fluid flow in thereservoir.

The three major types of enhanced oil recovery operations are chemical flooding (alkaline

flooding or micellar polymer flooding), miscible displacement (carbon dioxide [CO2] injection or

hydrocarbon injection), and thermal recovery (steam flood or in situ combustion). The optimal

application of each type depends on reservoir temperature, pressure, depth, net pay, permeability,

residual oil and water saturations, porosity and fluid properties

y y

Chemical techniques: utilize reagents that change the

physical properties of the produced fluid or the

displacement fluid.

Miscible processes: Miscible fluids are utilized to

produce oil that could potentially become residualoil. This is achieved by injecting a fluid that mixes

with the produced fluid. Typical miscible fluids are

organic-based solvents, CO2Thermal techniques: reduce the viscosity of oil

within the reservoir .

Thermally enhanced oil recovery methods (TEOR) are


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Thermally enhanced oil recovery methods (TEOR) are

tertiary recovery techniques that heat the oil, thus reducing its

viscosity and making it easier to extract.

Steam injection is the

most common form ofTEOR.

In-situ burning is another

form of TEOR, but

instead of steam, some of

the oil is burned to heatthe surrounding oil.

Hierarchy
http://upload.wikimedia.org/wikipedia/en/6/61/The_extraction_of_Oil_using_steam.jpg


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Meander belt(field)


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Pool

(Meander scroll)

Pool

Total volume in the

order of tens ofbillions of m3

Meander scroll(pool)


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Total volume in the

order of fewbillions of m3

Channel, point bar and crevasse-splay


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Total volume in the order of

several tens of millions of m3

5 orders of migrating and overlapping bedforms of varying type and dimensions are present


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Channel, point bar and crevasse-splay


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Total volume in the order of

several tens of millions of m3

Lobe sheet Level 4 heterogeneity


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Bedding unit


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Mud-laminae between and within bedsets are the result of flow separation and variation

in flow regime.


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Sech et al. (2009)


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Hierarchical orders of internal complexityobserved in the Torre Saracena section.

The main feature of the first-order units is


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alternation of thicker and thinner packages of

dunes. Each unit is, in turn, internally characterized

by alternating simple and compoundsecond-order foreset units. Tidal bundles are the

main features of the third-order units, whilst

bioclastic and siliciclastic packages of laminae

typify fourth-order units (see text

for explanation). Longhitano et al. (2010)

petroleum geology 10 11

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